1. Field of the Invention
The present invention relates to a cooker comprising a gas burner for heating food material in a container, temperature detection means for detecting the temperature of the bottom face of the container and issuing a temperature signal, and a heat control circuit for controlling the amount of heat issued from the gas burner based on said temperature signal.
With the term xe2x80x9ccookerxe2x80x9d we means all kind of cooking appliances that use a gas burner for heating/cooking a food material, cook tops, ranges and cooking hobs included.
The above kind of cookers does not need the presence of the user so that he does not need to check and to control the cooking process continuously. Several functions of the cooking process, for example to detect the boiling process, to control the boiling process, to control the simmering etc., can be automatically performed in a gas cooker by measuring the bottom temperature of the container or pan.
During heating and boiling process of a liquid (water) in a pan, the thermal content of the liquid itself and of the pot varies following some physical laws which depends mainly from the following parameters: liquid quantity and type, heat supply, room conditions (temperature and pressure), pan type.
A method for monitoring the thermal content of the foodstuff is to measure the temperature of the pan. In fact, while the absolute temperature of the pan bottom/sides depends on the thermal conductance of pan material and on heat supply, the temperature gradient is strictly dependent on the liquid content in most part of the heating process.
Furthermore when water starts to fully boil, both liquid and pan temperature reach a constant value.
As a consequence the boiling process can be monitored by simply measuring the pan temperature gradient, as the output to a known heating input (burner power).
2. Description of the Related Art
EP0690659 discloses the detection of pan sidewall temperature by means of an IR sensor placed on an electric hob. This sensor can allow the user to select the desired temperature food range and to maintain it during cooking process. This solution has the drawback that a special pot with a known emissivity material coating must be used. Furthermore, on a gas cook top the effect of exhaust gas lapping pan walls could represent a serious noise factor.
WO9719394 discloses a boiling detection and control device based on the thermal dynamic answer to modulated heat input. This solution implies the use of an electronic device to modulate the power supply (i.e. an electronic gas valve). Furthermore the mean heat supply during heating up process is less than the maximum available, thus increasing boiling time.
U.S. Pat. No. 5,310,110 discloses a boiling detection and control device based on the evaluation of the pan bottom temperature. Food quantity and type determination is made by evaluating temperature variation during last part of heating process, near incipient boiling. This phase strongly depends on how bubbles nucleate on the water-pan interface, so that the process is regulated by a lot of uncontrollable parameters (i.e. wettability of pan surface, calcareous deposit in the water, etc.). Furthermore burning prevention means are based on pre-set empirical data.
U.S. Pat. No. 4,646,963 discloses a boiling detection and control device based on the evaluation of the pan bottom temperature. The sensor is allocated in the burner cup, with its axis offset respect to the gas nozzle. A spring and the choice of material assure good mechanical and thermal contact between the pan and the temperature sensor. This solution has the drawback that the gas burner cannot be of a standard type, in fact this solution requires a special gas burner with a hole to permit the temperature sensor presence, and this means that this type of gas burner is expensive. An additional negative point is related to the fact that with the temperature sensor assembled in the burner itself, the measured temperature is largely influenced by the flame and by the high operating temperature of the burner cup.
A main object of the present invention is to provide a cooker of the type mentioned above which does not have the above drawbacks and which is simple and economical.
A cooker in accordance with the accompanying claims overcomes such drawbacks.
The temperature detecting means is a sensor device that can monitor the thermal status of the vessel, by a contact measurement and it is placed in a zone of the cooker around the burner and it is further shielded from the influence of the burner flame. The main advantage of the present invention is to avoid any influence on the temperature sensor device caused by the burner flame, such influence being mainly due to radiation and convection.
According to a first embodiment of the invention, the temperature sensor is placed inside a seat in the grids of the cooktop, thus avoiding any expensive modification to the burner structure and using the grid as a thermal shield for the temperature sensor.
The grids are preferably of the xe2x80x9cintegralxe2x80x9d type, i.e. are formed by the cooktop itself. They can be obtained by pressing the metal sheet forming the surface of the cooktop. The cooktop material can be glass or stainless steel or any other materials suitable for a high temperature range and for the needed structural specifications.
According to another embodiment, standard removable grids are used, with a wire or wireless connection between the temperature sensor and the heat control circuit of the cooker.
The temperature sensor can be any device reactive to pan thermal status: i.e. a thermistor or a thermocouple or thermocouple in an xe2x80x9copen configurationxe2x80x9d. The latter is a thermocouple whose two wires are separately in contact with the pan bottom: the signal is thus proportional to the voltage drop across the two wires and the pan metal material, all of them forming an electric circuit. This easily allows using the sensor both for thermal status monitoring and for pan detection.
Being the sensor placed in an area that is directly warmed either by the cooktop material or by the pan bottom, the sensor has to be designed in such a way to be thermally insulated from the cooktop. The gas flame heats the cooktop structure: its temperature variation follows a rise depending on hob material conductance and on convective heat exchange with air. Thus it is quite independent from the heating process of the foodstuff inside the pan. More precisely, the top of the grids is influenced both by the cooktop itself and by the pan, but its thermal history follows the pan variation temperature in a filtered way, i.e. by moving away a heated pan from the cooktop, the temperature of grids decrease but with a time lag and with an unpredictable amount.
The gas exhaust effect produces high noise in the temperature signal. The grids themselves protect and shield the sensor, by deviating the hot air flows and by shielding radiation from the burner.
According to another embodiment of the invention, few ports of the burner facing the temperature sensor are occluded. This can be easily done by having a sector of the flame spreader unit of the burner without any passage for the mixture primary air/gas. This occlusion minimizes the temperature effect produced by the flame or the exhaust gases over the temperature sensor.
Even if from tests carried out by the applicant the shielding effect of the grid or of the xe2x80x9cchokedxe2x80x9d burner is already sufficient to guarantee a reliable temperature signal to the heat control circuit, the present invention is intended to cover also a combination of a shielding grid and of a choked sector of the flame spreader unit of the burner.